The present invention relates to a simple and inexpensive power factor improving circuit, and more particularly, to a technique of a control circuit of the power factor improving circuit.
FIG. 1 is a block diagram of a conventional power factor improving circuit. In the power factor improving circuit shown in FIG. 1, a series circuit includes a step up reactor L1, a switch Q1 comprising an MOSFET and a current detecting resistor Rsh are connected to both output ends of a full-wave rectification circuit B which rectifies AC current of an AC power supply Vac. A series circuit includes a diode Do and a smoothing capacitor Co is connected to both ends (between a drain and a source) of the switch Q1. A load Ro is connected to both ends of the smoothing capacitor Co. The diode Do and the smoothing capacitor Co constitute a rectification smoothing circuit. The switch Q1 is turned ON/OFF by PWM control of a controller 10.
The current detecting resistor Rsh is connected between a negative side output end P2 of the full-wave rectification circuit B, one end of the switch Q1 and one end of the smoothing capacitor Co. The current detecting resistor Rsh detects input current conducting through the full-wave rectification circuit B.
The controller 10 includes an operational amplifier 11 as an output voltage detector, a multiplier 12, an operational amplifier 13 as a current detector, and a pulse width modulator 14.
The output voltage detector 11 amplifies a difference between voltage of the smoothing capacitor Co and a reference voltage Vref, generates an error voltage and outputs the same to the multiplier 12. The multiplier 12 multiplies the error voltage sent from the output voltage detector 11 by a full-wave rectification voltage sent from a positive output end P1 of the full-wave rectification circuit B, and outputs the multiplied output voltage to a current detector 13.
The current detector 13 amplifies a difference between voltage which is proportional to input current detected by the current detecting resistor Rsh and a multiplied output voltage from the multiplier 12, generates error voltage, and outputs the error voltage to the pulse width modulator 14 as a comparison input signal.
The pulse width modulator 14 inputs a triangular wave signal and the comparison input signal from the current detector 13. The pulse width modulator 14 generates a pulse signal which becomes ON as the value of the comparison input signal becomes equal to or higher than a triangular wave signal and which becomes OFF as the value of the comparison input signal becomes less than the triangular wave signal, and the pulse width modulator 14 applies the pulse signal to a gate of the switch Q1.
Full-wave rectification voltage which is obtained by rectifying input voltage (AC current) of the AC power supply Vac by the full-wave rectification circuit B has a shape of sinusoidal wave which is inverted every half-cycle (half-cycle sinusoidal wave, hereinafter). The multiplier 12 inputs half-cycle sinusoidal wave voltage from the full-wave rectification circuit B, and inputs voltage from the output voltage detector 11, and multiplies these two voltages to change the magnitude of the sinusoidal wave and outputs the same. The current detector 13 compares the half-cycle sinusoidal wave voltage from the full-wave rectification circuit B with voltage Vrsh generated in the current detecting resistor Rsh which is proportional to the input current, and controls such that the input current becomes the half-cycle sinusoidal wave. Therefore, the input current conducting through the current detecting resistor Rsh can be changed to sinusoidal wave similar to the input voltage of the AC power supply Vac every half-cycle and thus, the power factor can be improved.
The operation of the power factor improving circuit having such a structure will be explained. As the switch Q1 is turned ON, current conducts through B→L1→Q1→Rsh. This current is straightly increased with time.
Next, as the switch Q1 is changed from ON to OFF, voltage of the switch Q1 is increased by voltage induced by the step up reactor L1. Further, since the switch Q1 is turned OFF, current conducts through the switch Q1 becomes zero. Thus current conducts through L1→Do→Co, and electricity is supplied to the load Ro.